Grazing-incidence electron microscopy of surface blisters in single- and polycrystalline tungsten formed by H+, D+ and He+ irradiation

Blisters on single- and polycrystalline tungsten surfaces formed by hydrogen and helium ion irradiation were investigated by grazing-incidence electron microscopy (GIEM) with an ultra-high-voltage transmission electron microscope. It was found that the blister skin thickness formed by D+ irradiation of polycrystalline tungsten (PCW) was considerably larger than the calculated ion range of the implants; however, this skin thickness (or blister depth) is not related to the pre-existing grain boundaries in the PCW. Blister formation was also observed with GIEM for single crystal tungsten (SCW) irradiated with H+, D+, and He+. The critical ion fluence for blister formation in SCW is estimated to be ∼1023 H+(D+)/m2 for H(D) and ∼1021 He+/m2 for He. The size of the blisters and their skin structure depends on the irradiating conditions. Typical skin thickness was about 50-150 nm. Based on the assumption that gas particles (H2, D2, and He) accumulate within the blisters during H+, D+, and He+ irradiation, the GIEM measurements provide a means to derive an estimate of the amount of gas so accumulated, by reproducing the observed blister shapes with finite element method (FEM) calculations. From the GIEM images and FEM calculations we have estimated the number of implanted ions being retained in the blisters, and compared these amounts with published retention measurements. A mechanism for the blister formation is proposed based on the present results.